Biofuel for thought

For the first time, French scientists have used NIR spectroscopy to monitor the transesterification reactions that produce biodiesel in a microreactor.

These transesterification reactions occur when plant oils or animal fats are mixed with an alcohol, usually methanol or ethanol, in the presence of a catalyst, usually a liquid acid or base. The end result is the fatty acid methyl or ethyl esters that make up biodiesel, together with glycerol as the main by-product.

There are many factors that influence the efficiency of these transesterification reactions, including the temperature, water content, type of catalyst, and oil to alcohol ratio. So it's often useful to be able to monitor the reaction process as these factors are varied, in order to find the best combination. This monitoring has previously been done using analytical techniques such as gas chromatography and high performance liquid chromatography.

The problem with these techniques, though, is that they can only be performed off-line, which means taking regular samples from the reaction chamber for later analysis. This is fine if the process is taking place in a batch reactor, where samples can be analysed between batches, but not if it is taking place continuously in a microreactor.

So when looking to conduct on-line monitoring of the transesterification reactions between sunflower oil and ethanol in a microreactor, French scientists led by Sophie Thiebaud-Roux at INP-ENSIACET in Toulouse decided to try NIR spectroscopy. Although NIR spectroscopy had previously been used to monitor biodiesel production, it had never before been used to monitor the continuous transesterification reactions that take place in a microreactor.

As a first test, Thiebaud-Roux used NIR spectroscopy to monitor how changing the ratio of oil to ethanol affected the production of ethyl oleate, which is the main component of this kind of biodiesel, and it turned out to work very well. After first calibrating the NIR spectral signal using measurements of ethyl oleate concentrations made by a gas chromatograph, they found that NIR spectroscopy could quickly and accurately measure the changing concentrations of ethyl oleate produced by the microreactor.

Reporting this work in the journal Fuel, Thiebaud-Roux concludes that NIR spectroscopy offers a “fast, safe, reliable, non-destructive and inexpensive” method for monitoring biodiesel production in microreactors.